Sinusoidal oscillators consist of two basic parts, a passive filter and an amplifier. The passive filter determines the frequency of operation of the circuit. The amplifier imparts energy into the filter as needed to sustain oscillations. The amplifier gain is sufficient for oscillation to commence and build up until non-linearities limit the amplitude. The oscillator circuit is usually designed so that the amplifier has minimal effect on the frequency of operation.
The problem with sinusoidal oscillators is that when the amplifier is initially turned on, the onset and build up of oscillation occurs gradually. Many cycles are required before steady state operation is reached. For some applications, this condition is acceptable, but for others such frequency and amplitude variations are intolerable.
For example, high speed printers use an oscillator in the printhead to clock pel signals. Problems occur when either the page-per-minute rate or the pel-per-inch rate becomes so high that a clock rate from a standard oscillator is not sufficient. That is, the clock rate is insufficient if there is too great a transient period in the start up or in the noise factor that creates a variable period between clock pulses. If the oscillator is instantly started at the beginning of a scan, pel placement accuracy and system response time are both improved.
Various prior art circuits have addressed improvement in the stabilization of start up frequencies for oscillators. One approach is to store the tank energy at the time of gate sampling for reapplication on start up. U.S. Pat. No. 3,646,477 by Goodall shows such a circuit.
Yet another approach is to expedite charging of the tank circuit when the circuit is turned on and/or to enhance collapse of the tank circuit oscillations when it is turned off. This is the approach followed in U.S. Pat. Nos. 3,991,388 by Harshbarger, 3,363,198 by Davies and 4,543,527 by Schuchmann et al.
U.S. Pat. No. 3,229,227 by Popodi is concerned with reducing transient effects on oscillator start up and shut down in response to gating pulses. The circuit establishes current conditions so that each initial output pulse of the oscillator starts at the same magnitude of voltage; namely, the peak voltage of the inductor element.
None of the prior art teaches start up and stopping of an oscillator circuit so that the passive filter constantly functions at steady state conditions.